Systems are known in the electrical discharge machining art in which electrical gap parameters, most often average gap voltage or gap peak voltage, are responded to when these are of an abnormal level to provide interruption of the power supply or reduction of its power output. Electrical discharge machining power supplies must have the capability of being operated in either standard polarity with the electrode negative and workpiece positive or, in reverse polarity in which the electrode is positive and the workpiece negative. This choice is made depending on the particular materials used for the workpiece and the electrode.
Protection circuits used in the power supply must be readily adaptable to changeover between the two gap polarities. In the prior art, a number of relay switching systems have been used. One example of such a system is described and shown in U.S. Pat. No. 3,775,578 issued to Dalton R Verner on Nov. 27, 1973 for "Polarity Reversal System for Electrical Discharge Machining Apparatus". Another solution has been to take signals of the two polarities, amplify these and then pass the signals through a relatively complicated absolute value circuit.